Well, all of the 3GPP mobile networks switch to a different logic for emergency calls. In the GSM case all emergency calls (112 is hardcoded in the specification and there is a provision for both USIM and the network to add more numbers that behave that way) use different RR layer protocol that deals in physical addresses (ie. IMEI) and the whole process is streamlined. The MS that initiates emergency call will just uplink an emergency RACH frame to anything that it is synchronized with and the network will respond by allocating traffic channel for that, there is no kind of GSM signaling nonsense with multiple packets involved in that.